To take advantage of the best prices you will need to prove you are a leisure traveller, en route to heathrow airport, so you will need to provide either yr flight details or yr holiday booking reference. I don't think you will need to provide this when you arrive at the station but you may be asked to give them on your travels from the station to the airport.
Going to the airport on the train is great, you can take a nap, read a book (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these cheap tickets from northampton station to the airport is that there must be at least two grown ups travelling together. But another great advantage is that children go free. All kids under 5 go free, and then one child over five but under 16 per adult travelling by train to heathrow airport.
be sure to leave plenty of time to get to the airport early enough to eat, shop and so on, so check the times of trains from northampton station before you go. If you end up with time to kill at heathrow airport you can always read a book or play travel scrabble again! As you have bought yr train ticket in advance, you don't need to worry about the opening hours of northampton station.
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